The present invention relates to a unique security control circuit for preventing the unauthorized use of a semiconductor memory incorporated within a microprocessor unit. Presently there is a need to protect programs and data stored in memory devices of data processing equipment from being read by unauthorized persons. With increasing emphasis towards software programs, the need to protect such programs also increases. Once a program or other data has been stored in a memory device, it is very easy for one to remove the memory device and copy the program from the memory device using well known circuits. Thus it is the primary object of this invention to provide a semiconductor memory unit which requires the entrance into the memory unit of a predetermined sequence of data characters before the memory unit can be accessed. It is a further object of this invention to provide a control circuit located within the semiconductor memory unit for enabling the memory unit upon receiving a predetermined sequence of data characters. It is another object of this invention to provide such a control circuit which is simple in construction and low in cost.